Final answer:
A dangerous condition on the subway system that a trackman should observe and report to his superior is any situation that poses a threat to passenger safety or the operation of the subway.
Step-by-step explanation:
A dangerous condition existing on the subway system which a trackman should observe and report to his superior would be a situation that poses a threat to the safety of passengers or the operation of the subway. This could include things like damaged tracks, malfunctioning signals, or any other hazards that could result in accidents or delays. For example, if a trackman notices a crack in the rail or a broken signal, they should immediately report it to their superior so that appropriate action can be taken to address the issue and ensure the safety of everyone using the subway system.